The OFDM (Orthogonal Frequency Division Multiplexing) technique is a powerful transmission technique in the field of Powerline Communication (PLC); that is to say, of signal transmissions on low-voltage networks. Frequency-selective channel properties that are detected analytically by an appropriate transfer function and are described uniquely are established virtually regularly in the PLC transmission channel via reflections that occur. An important reason for the selection of an OFDM transmission technique resides in the robust system behavior in the frequency-selective channels, and in the relatively low outlay on processing for equalizing the received signals.
An interference effect that is, in particular, typical of PLC applications but is not unique occurs on the low-voltage network; for example, owing to periodic switching processes caused by switched mode power supplies. This physical process generates short-term, harmonic, that is to say narrowband interference signals that are superimposed additively on the useful signal. These interference signals are also designated as “Inter-Carrier Interference”, “Adjacent Sub-Carrier Interference” (AsCI) and “Adjacent Carrier Interference” (ACI), and negatively influence at the receiving end the transmission quality (residual/bit error rate) of a relevant data transmission system that is based on the OFDM technique. Such AsCI effects are generally produced by additive stationary harmonic interference components on the transmission channel, by Doppler interference components that arise as a consequence of a moving receiver, or else by phase noise processes of the receiver oscillator, whose interference spectrum contains significant energy components outside the OFDM subcarrier bandwidth.
In the past, AsCI in OFDM systems were either accepted, that is to say no compensation was performed, or were compensated by expensive equalizers and/or so-called channel tracking methods in the case of time-variant interference effects (for example, Doppler effect on the channel or phase noise present).
A known so-called windowing used in this case has so far been used solely at the transmitting end for the purpose of spectrum shaping, and at the receiving end for the purpose of improving the adjacent channel suppression. Document EP 0 802 649 A1, for example, has disclosed an application of windowing for reducing so-called spectral leakages and, thus, for adjacent channel suppression.
It is an object of the present invention, therefore, to reduce negative influencing, occurring at a receiving end, of the transmission quality (residual/bit error rate) of a data transmission system based on the OFDM technique via narrowband harmonic interference signals, and thereby to raise the transmission quality of the data transmission system.